Process of making soft iron shot



Jan. 6, 1959 L. P. WILSON ETAL 2,367,554

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INVENTORS; Lew/5 P. WILSON &

ROBERT L .TURNE/E ,DEC'Q ATTORNEYS.

United States Patent C PROCESS OF MAKING SOFT IRON SHOT Lewis P. Wilson, Edwardsville Township, Madison County, Ill., and Robert L. Turner, deceased, late of Alton, 111., by Dorothy H. Turner, executrix, New Haven County, Conn., assignors to 01in Mathieson Chemical Corporation, a corporation of Virginia Application April 20, 1953, Serial No. 349,612

6 Claims. (Cl. 1483) This invention relates to the treatment of iron shot, and particularly to a process of manufacturing the same with properties such that they are adapted for use as the projectile charge in shot gun ammunition. This application is a continuation in part of our co-pending application, Serial No. 130,122, now abandoned.

For many years, the bulk of projectiles made by the sporting ammunition industry has consisted of lead or its alloys in spite of known disadvantages and undesirable features of the use of the metal lead. One disadvantage is the tendency of a lead projectile, when subjected to the heat and driving force of the burning powder, to foul or lead the barrel, and is due to the exceptional softness of the metal and its low melting point. Another disadvantage of the use of an exceedingly soft metal, such as lead shot, is that the impact of initially spherical pellets of lead with each other and the interior of a gun barrel causes deformation of the shot, which then pass through the air toward the target with unequal velocities in a lengthening shot string, which is not as effective on a swiftly moving target as a short shot string. 'Still another disadvantage of the use of lead is its great density, which makes lead unsuitable for use as a high velocity projectile designed primarily for exceptional effectiveness at short ranges. Yet another disadvantage of the use of lead, particularly as shot, is the poisonous nature of the metal, which not only causes contamination of game in which such shot has lodged, but also will cause the death of wild fowl inhabiting feeding grounds on which shot has accumulated over a period of many years. Furthermore, the high cost of lead, as compared to the cost of other common metals and materials, is a serious economic disadvantage.

Attempts to overcome the disadvantages of lead shot have been made. For example, soft lead shot have been plated with various metals, such as tin, zinc, and copper, to prevent shot deformation and barrel leading. In other instances, hard lead alloys, or lead alloys amenable to hardening, have been used to produce chilled lead shot. The above-noted and similar treatments have not avoided the use of lead and in many instances are attended by the cost of additional steps and equipment.

Although iron shot was suggested as a projectile as early as the beginning of the fifteenth century, and although, as early as the year 1880, the'Winth'erliches, aware of the economic advantage of using for-sporting ammunition a dropped shot of iron" instead of lead, obtained two United States patents, namely, No. 224,858 and No. 236,134, in which disclosure is made of a method of manufacture and the advantages of iron shot, these early developments have not been followed by widespread use of iron shot. The potential advantages of iron shot have never been realized largely because iron shot, as produced heretofore, have been so hard as to damage the gun barrels. Even the hardened gun barrels of the present day erode seriously when used with iron shot of the character heretofore suggested. The damage is accentuated since the introduction of the use of smokeless powder and high velocity ammunition. While the method of making spherical pellets, comprising quenching drops of molten metal, is the most feasible commercial method of making shot, it is attended by most severe chilling which, in the manufacture of iron shot, consists in a sudden drop from'a temperature above 2800 Frto room temperature, producing iron pellets much too hard for use in ordinary shot guns.

Treatment of ordinary iron shot by the usual annealing techniques doesnot produce the characteristics necessary for a satisfactory projectile.

Heretofore it has been proposed to produce steel articles having a low order of hardness from a steel casting or ingot by mechanically-working the metal in a series of steps followed byintermediate-annealing steps, and, prior to the final working step, heat treating in a decarburizing atmosphere. If the final working step was of such character as not to harden the metal appreciably, no final annealing is necessary. The requirement for mechanically working, however, eliminates the possibility of utilizing such a process upon parts so small as shot.

It has also been proposed to treat iron (with or without appreciable amounts of other metals) sheets, bars, or rods, either to decarburize them or to improve their magnetic properties, by heating in a hydrogen atmosphere, to a temperature of between 900 C. (1652 F.) and 1400 C. (2552 F.), but again these techniques, which essentially include the mechanical working as an antecedent to the heat treatment, are inapplicable to shot, and at the higher temperatures the shot will deform.

Decarburization ofunworked castings in the same temperature range and in a wet hydrogen atmosphere has also been proposed (U. S. Patent No. 2,225,968), but the efficacy of such treatment-has been dependent upon the presence of other metals, such as copper and vanadium-- and required on the order. of forty hours or more. The presence of the other metals not only renders iron shot unfit for projectile use, but the duration of the treatment is so prolonged that its applicationto shot would cost more than the shot is worth.

-It is, therefore, the object of the present invention, generally stated, to produce iron shot suitable for use as the projectile in shot guns, and which will not abrade or otherwise damage the barrel of-the shot gun.

Another object of the invention is to provide such shot having physical properties ofa character such as to yield desirable ballistic results.

It is also an object of the invention to provide an apparatus suitable for carrying out the treatment of the small iron castings.

Other objects will become apparent to those skilled in the art as the following description is read:

In accordance with the present invention, shot, which is unabrasive to gun barrels and which has quite satisfactory ballistic characteristics, is cast from substantially pure iron by allowing the globules of the molten metal to fall in air where the globules assume a spherical shape before they fall into a tank of water, while still intensely hot, whereby they become solidified and then (without any mechanical working). heat-treated in a hydrogen atmosphere at a temperature in excess of the A transformation point, but below the A; transformation point, so that the resulting shot have a hardness of less than D. P. H., and whose hardness at their centers is not.sub stantially greater than their surface hardness. f-The low order of hardnessat the surface minimizes the-tendency to score or scratch shot gun barrels, and the softness of the interior is important from the standpoint of v avoid ing damage to the choke of the gun. By substantially pure iron, as the expression is used herein, is meant iron which is devoid of deliberate addition of hardening elements, such as chromium, molybdenum, tungsten, nickel, copper, vanadium, etc., and which is as free as practicable from other elements. Such substantially pure iron will usually contain less than a quarter of a percent of impuri- Adaptation of the Tukon Tester and the Knoop Indenter, and as described in the February 1949, Issue of Modern Machine Shop in the article entitled Microhardness Testing of Small Tools by G. E. Shubrooks. On this scale, hard lead, for example, has a hardness value of about 35, zinc and copper have hardness values of from 50 to 60, gun barrels of mild steel have hardness values of from 100 to 130, and chrome molybdenum gun barrels have a hardness of about 240. The hardness values of the iron shot herein referred to were determined after conditioning the shot as follows:

The shot samples were mounted in a methyl methacrylate polymeride plastic, such as Lucite, and the mounting was ground down on an abrasive wheel until approximately one-half the diameter of the mounted shot was removed. The mounting was then polished on abrasive paper having grits 0, 2-0, 3-0, and 4-0. The specimen was then polished on a wheel covered with billiard cloth, while the wheel was kept wet with a liquid abrasive, such as an alumina. After all noticeable scratches were removed, the sample was etched with an acidic solution, such as 2% Nital" solution. repolished on a wheel covered with Selvyt cloth using the liquid abrasive. After this final polishing, the sample was etched once more and was then ready for hardness testing.

In order to produce iron shot having physical properties best suited for projectile use, we have found that the micro structure of the finished shot should be coarsegrained and in a single phase. For universal use, i. e., in old fashioned, as well as in modern shot guns, the grain size of the finished shot should be no smaller than ASTM grain size No. 2 (ASTM standards, 1944, p. 1933).

and preferably on the order of Nos. and 0, but when the shot are to be used only in guns whose barrels have the hardness of 125 D. P. H. or higher, grain size as small as-ASTM No. 6 may be tolerated.

We have discovered that the desired properties may be imparted to shot as cast from substantially pure iron by heat treatment in a non-oxidizing hydrogen atmosphere above the A point, and that desirable properties are developed in such shot without prolonged heat treatment. In practice, the time period above the A point may be as short as 30 seconds, but in commercial scale operations five minutes provides a more adequate factor of safety to assure that the centers of the shot have reached the A point. It is essential, however, to avoid heating the shot as high as the A; transformation point, and therefore the temperature of heat treatment should not exceed about 2400 F. It is also important that the shot be cooled down to about 1300 F. before they are removed from the hydrogen atmosphere. Indeed, unless other precautions against oxidation and scaling are taken, it is preferable that the shot be cooled to room temperature or nearly so while enveloped in hydrogen.

In order to achieve the desired physical properties for projectile purposes, it is important that the shot be cast from iron which is substantially pure, so that, when introduced into the heat treatment without previous mechanical working, the iron contains not more than 0.25% of impurities other than carbon and nitrogen. We have The specimen was then observed that the nitrogen content of the shot, as cast; has an important bearing on duration and effect of the heat treatmentas with shot containing .019.020% nitrogen, as cast, the ultimate shot are somewhat harder and their grain structure smaller than shot containing .008.010% nitrogen as cast.

The heat treatment is carried out in a hydrogen atmosphere as dry (i. e., free of water or water vapor) as possible (in contradistinction to the known wet hydrogen annealing techniques wherein enough water is introduced to create an oxidizing atmosphere). Some water will inevitably find its way into the heat-treating atmosphere, either as an impurity in the hydrogen supply or as a reaction product from any oxide which may be present on the shot being treated, but so long as the content of water in the treating atmosphere does not exceed about 3%, the atmosphere is considered non-oxidizing and desired results are achieved.

In general, the treatment of iron shot according to this invention involves first providing the as-cast shot (near spheres most of whose diameters are between 0.062 and 0.144 inch) having the following typical analysis:

Percent Nitrogen 0.0000.020 Phosphorus and sulphur 0..0070.030 Manganese and/or silicon 0.008-0.115 Carbon 0.0l5-0.l00 Iron Balance If it is desired to produce shot having a hardness consistently well below D. P. H. with a grain size larger than ASTM No. 2, it is important that the nitrogen content of the shot, as cast, not exceed about 0.012%. By melting the metal in a vacuum furnace, and otherwise controlling the conditions so that the nitrogen content is 0 (or so close to it that it is unmeasurable), a hardness as low as 60 D. P. H. may be achieved with a grain size of ASTM N0. 00.

Having provided the substantially pure iron shot with the lowest feasible nitrogen content as cast, the shot are then heated in a hydrogen atmosphere and maintained at a temperature above the A point, but below the A, point, for the time period above-indicated, which may be as short as 30 seconds or longer, depending upon the apparatus employed and otherenvironmental conditions consistent with the rule that the higher the temperature (below the A; point) and the lower the nitrogen content of the as-cast shot, the shorter the time period of treatment above the A point need be.

Having elevated the individual shot to a temperature above the A point, while enveloped in the hydrogen, the shot are then quenched in a hydrogen bath and preferably maintained in such hydrogen bath until the temperature of the individual shot descends to 300 F. or less.

The treatment may be carried out either as a batch process or as a continuous process.

To assure that the individual shot constituting the charge be uniformly treated, and to minimize the tendency thereof to deform, sinter, or cluster at the temperature of treatment, it is important that the temperature not reach the A; point, and it is desirable to keep the shot in motion during the heat treatment.

In our parent application, above-identified, the results given were obtained from batch operations upon shot whose composition was as indicated in the foregoing table, except that the nitrogen content was within the limits of 0.008 to 0.012%; and the shot were maintained at a temperature of 1675 and 1800" F. for periods of 6.0, 90, 120, and 180 minutes. At the end of the heat treatment, the shot were cooled by the continued introduc tion of hydrogen until their temperature reached 212 F. or less. The results obtained with four separate batches of shot indicated A, B, C, and D, respectively, were as follows;

easiest an E3 TREATMENT Tim'et Hydrogen, Temp., ultimate cu. ft./ Batch F. temp. hour/lb.

(minutes) (measured at 70 F.)

SHOT

Before treatment D. P. hardness Grain Batch Center Surface size Range Average Range Average SHOT After treatment D. P. hardness Grain Batch Center Surface size Range Average Range Average The following table typifies the chemical analyses of the shot before and after treatment, the results applying specifically to shot from batch A in the foregoing tables:

Before After treatment treatment Percent Percent 0. 009 0. 009 None None 0. 017 0. 013 0. 03 0. 030 0. 007 0. 002 0. 050 0. 008 Balance Balance Spectographic analysis of the same sample revealed the presence of traces of nickel, molybdenum, aluminum, vanadium, and copper, both before and after treatment. In other samples, traces of chromium, tungsten, and cobalt were found. In any case, the quantities of trace elements are too minute to measure and do not affect the properties of the shot.

Subsequent investigations, in which we used a furnace constructed and arranged for the continuous treatment of shot, have indicated that the time periods of treatment utilized in the batch operations were considerably longer than necessary. Utilizing a shaker-hearth type furnace of the character hereinafter described more in detail, we have ascertained that shot of the same composition and physical properties as that of batches A, B, C, and D above could be treated, in a hydrogen atmosphere, in an elapsed time (from room'temperature into the furnace to room temperature out of the furnace) of fourteen minutes where the maximum temperature at- 6 tained by shot in the furnace was 1850 F., with the following results:

To establish the fact that the changes in physical properties, as well as the reduction of nitrogen and carbon, begin practically instantaneously, a series of shot (batches F, G and H) were passed through the furnace at an accelerated rate (considerably in excess of the rate at which such shot traveled under the maximum speed attainable with the shaker mechanism) so that the time at temperature above 1670 F. was 30, 60 and 300 seconds, respectively. The shot in each batch had average, as cast, D. P.'H. values of: Edge 116, center 121, and were all of the same composition, being substantially pure iron In the foregoing examples indicated, batches A to H inclusive, the shot were cast from substantially pure iron melted in a vacuum furnace so as to minimize the nitrogen absorption of the melt. Accordingly, the as-cast shot have lower nitrogen content, being on the order of 0.008-0.010% as indicated, than would be the case when the shot are cast from a melt made in an electric furnace or open hearth. The invention is, however, applicable to the treatment of shot having higher as cast nitrogen contents, such as those on the order of 0.019

to 0.020% resulting from an electric furnace melt of substantially pure iron. As illustrating the results of continuous hydrogen-atmosphere heat treatment in the shaker-hearth type of furnace, with an elapsed time of fourteen minutes, upon shot whose nitrogen content was on the order of 0.019 to 0.020%, but otherwise of the same composition as that described above (said shot having been cast from a melt made in an electric arc furnace and having an as cast hardness of 260-350 D.

P. H.) the following results were attained:

D. P. hardness Grain Batch Center Surface size Range Average Range Average In the accompanying drawings, Figure l is a diagrammatic view of the shaker-hearth type of furnace with quenching attachment to which reference has been made above;

Figure 2 is a diagrammatic view of a vertical type furnace suitable for use in accordance with the present invention; a

Figure 3 is a photomicrograph at 100x magnificatio of the grain structure and individual, as cast, shot before treatment; and

Figure 4 is a photomicrograph at l magnification of the grain structure of an individual shot after treatment.

Referring now to Figure l of the drawings for an illustrative embodiment of the shaker-hearth type furnace hereinbefore referred to, the structure may consist of concentric tubular shells 1 and 2, between which an electric heating element 3 is disposed. The heating element may be either an electrical resistance or an induction coil, but in either case, must be capable of elevating the temperature of shot pellets within tube 2 to a temperature above 1670 F., and preferably as high as 1850" F. or 2200 F.

The inner tube 2 is open at one end 4 and is provided with an opening 5 near its opposite end. The tube 2 is preferably formed of refractory material, and the exterior tube 1 may likewise be formed of refractory material.

Extending into the tube 2 through the open end 4 thereof is an oscillatable hearth 6, the inner end of which is mounted upon a suitable cradle 7, and the outer end of which is connected to be driven by suitable shaker mechanism 8. The shaker mechanism is of a character such as to move hearth 6 toward opening 5 of the furnace gradually, and then jerk it in the opposite direction, whereby shot deposited toward the outer end of the hearth 6 are moved step by step toward the inner end of the hearth. Above the outer end of the hearth, a feed hopper 9 is provided for depositing the shot to be treated upon the outer end of the hearth 6.

The exterior tube is provided with an opening 11 in radial alignment with opening 5 in the interior tube 2, and a sleeve 12 extends across the space between the openings 5 and 11, thus providing a conduit. Beneath opening 11, a conduit 13 extends downwardly and constitutes the inner wall of a cooling device 14, whose outer wall 15 is spaced from conduit 13 and provided with a coolant inlet 16 and a coolant outlet 17. The conduit 13 projects beyond the cooling element 14 and is provided with a removable gas-tight closure 18. Between the closure 18 and the cooling element 14, the conduit 13 is provided with a hydrogen inlet 19.

Water, oil, cold air, or any other heat exchange medium may be utilized as the coolant and circulated through the chamber defined between the inner and outer walls 13 and 15 of cooling element 14. In lieu of providing a gastight closure at the lower end of conduit 13, the end thereof may be reduced and left open provided the vol-.

ume of hydrogen gas introduced at 19 is sufiicient to prevent the intake of air at the open end of conduit 13. An.

economy in hydrogen gas is, however, effected by making cooling element 14 of sufiicient volume to accommodate the product of a relatively long run of the furnace and then periodically discontinuing the feed of shot to the furnace while the treated shot within cooling element 14 are discharged therefrom by removal of cap 18. i

In the operation of the furnace, the shot deposited upon hearth 6 attain a temperature of at least1670 F. before they fall from the inner end of the hearth. When the shot fall from the inner end of the hearth, they are re-;

through the furnace to opening 4, where it may be burned An alternative form of furnace is shown in Figure 2 Hydrogen is continand consists essentially of a vertically arranged induction furnace having an interior induction chamber 21 provided with a spiral ramp 22 to retard the descent of shot deposited in the top of the furnace. The inclination of the ramp and the height of the furnace are coordinated with the heating capacity thereof so as to assure that individual shot pellets will be elevated to a temperature of at least 1670 F., and preferably 1850 to 2200 F., as they roll down the spiral ramp 22. From the bottom of chamber 21, the shot are dropped into conduit 13 leading to a cooling element of the character hereinabove described. As in the previous embodiment, hydrogen is continuously introduced so as to completely envelop the shot in the cooling element, as well as in conduit 13, and in the heating chamber 21, of the furnace. In order to eliminate any stack effect in chamber 21, which might result in an impoverished hydrogen atmosphere about the shot traveling down the spiral 22, a ceramic core 23 may be provided to fill the space between the inner edges of the spiral ramp. Thus, the hydrogen stream i compelled to follow the spiral path in countercurrent relationship to the downward rolling of the shot.

The apparatus above described provides for the continuous treatment of such shot at any chosen temperature between the A and A points, and within a considerable range of treatment time periods. As hereinbefore indicated, the essential feature of the invention is the provision of iron shot which will not erode gun barrels, and while we have herein recited the value of D. P. H. as the maximum hardness-which should be tolerated immediately after the heat treatment, it is to be noted that shot, cast from substantially pure iron, age-harden at the rate of about 10% over the first five months, after their treatment according to this invention. Thus a batch of substantially pure iron shot, containing 0.006% carbon and 0.008% nitrogen, before treatment in accordance with the present invention, had their carbon and nitrogen contents reduced, respectively, to 0.001% and 0.006% during treatment for thirty minutes at 1850 F., and at the end of such treatment, the average edge hardness was 78.1 D. P. H., and the average center hardness, 80.2 D. P. H. After the shot had aged for five months, however, the edge hardness average had increased to 88.0 D. P. H., and the center hardness to 90.0 D. P. H. Thus, by setting a maximum D. P. H. value of 110 for the treated shot, immediately after they are annealed, it is contemplated that the ultimate hardness, after aging a few months, may reach a value of D. P. H., and such is therefore contemplated by the invention.

A further factor enters into the selection of a critical maximum hardness value, and that is the size of the shot. With No. 9 shot (the smallest size used in commercial shotshells), a hardness as high as 120 D. P. H. is quite suitable and has no serious deleterious effect upon the barrels of the firearms. At the other extreme, No. 4 shot (which is the largest of the popular commercial sizes used in shotshells) exhibits quite a tendency to erode comparable gun barrels at a hardness as high as 100 D. P. H. Consequently, for No. 4 shot, it is preferred that the maximum value he 80 D. P. H. (prior to age-hardening) and the other sizes, between Nos. 4 and 9, may have their hardness graduated between the values indicated for Nos. 9 and 4. Accordingly, the time period of heat treatment between the A and A points is necessarily longer with larger shot than with smaller shot, or in the alternative, the temperature of the treatment'is higher with the larger shot in order that the centers thereof be heated at least to the A point.

From the foregoing description, it is believed that those skilled in the art will readily understand the principles of theinvention and its mode of operation, and the results accomplished thereby. While a variety of data have been given, showing the flexibility of the process, it is to be distinctly understood that the invention is not limited to the time periods or temperatures of treatment cited for illustration herein above, but that such variations and modifications thereof as may present themselves to those skilled in the art, without departing from the spirit of theinvention, are, although not specifically described herein, contemplated by and within the scope of the appended claims.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

1. In the art of making shot, the process comprising providing molten iron containing not more than about 0.25 percent impurities, casting the molten iron into near-spheres having a diameter between about 0.062 and about 0.144 inch, then heating the near-spheres in their as-cast condition above 1670 F. but substantially below the melting point thereof for about minutes, and maintaining a non-oxidizing atmosphere of dry hydrogen containing not more than about 3 percent water about said near-spheres while the same are being elevated to, maintained at, and cooled from said temperature to a temperature not in excess of 1300 F. 2. The process of claim 1 wherein the near-spheres are maintained at said temperature until the carbon content thereof is reduced to 0.008 percent or below and the nitrogen is reduced to 0.010 percent or below.

3. The process of claim 1 wherein the near-spheres are continuously agitated and enveloped in an atmosphere of hydrogen throughout the period during which they are being heated.

4. In the art of softening shot as cast from substan-. tially pure iron, the process comprising heating the shot in a dry hydrogen atmosphere containing not more than about 3 percent water to a temperature between 1670 F. and 2400 F., the shot being maintained above 1670 F. for not more than about 5 minutes, and, when the 10 shot reach the desired temperature, immediately quenching them in cool hydrogen to a temperature of 1300 F. or below.

5. The process of claim 4 wherein the cool hydrogen is passed over the quenching shot and from thence passed over the shot being heated.

6. In the manufacture of iron shot, the process comprising melting substantially pure iron in an atmosphere References Cited in the file of this patent UNITED STATES PATENTS 236,134 Wintherlich et a1 Dec. 28, 1880 1,760,241 Louenstein May 27, 1930 1,988,910 Merica Jan. 22, 1935 2,201,181 Kalling May 21, 1940 2,225,968 Boegehold Dec. 24, 1940 2,231,120 Herrmann Feb. 11, 1941 2,573,935 Stutzman Nov. 6, 1951 OTHER REFERENCES The Journal of The Iron and Steel Institute, No. 2, 1919, pages 408-413.

Controlled Atmospheres for the Heat Treatment of Metals, by Jenkins. Published by Chapman and Hall, Ltd., 1946, pages 276-278. 

1. IN THE ART OF MAKING SHOT, THE PROCESS COMPRISING PROVIDING MOLTEN IRON CONTAINING NOT MORE THAN ABOUT 0.25 PERCENT IMPURITIES, CASTING THE MOLTEN IRON INTO NEAR-SPHERES HAVING A DIAMETER BETWEEN ABOUT 0.062 AND ABOUT 0.144 INCH, THEN HEATING THE NEAR-SPHERES IN THEIR AS-CAST CONDITION ABOVE 1670* F. BUT SUBSTANTIALLY BELOW THE MELTING POINT THEREOF FOR ABOUT 5 MINUTES, AND MAINTAINING A NON-OXIDIZING ATMOSPHERE OF DRY HYDROGEN CONTAINING NOT MORE THAN ABOUT 3 PERCENT WATER 